The use of lead as a sheathing material around a hose for the curing process has been known for at least sixty years. Inherent in the use of lead in industrial hose applications are high energy, labor, and equipment costs, as well as certain environmental and health concerns. It was therefore desired to provide a less expensive and safer means of producing sheath cured hose.
It has been known to use a fabric wrapper applied under tension about unvulcanized articles supported by drums, molds, cores or mandrels, wherein the wrapper fabric is composed of nylon warp thread applied circumferentially to the unvulcanized article. It was recognized that the nylon material had been found to contract by a much greater amount when heated than cotton material, thereby providing substantially greater circumferential contraction.
There was also known a process for making a hose wherein an internally supported hose is drawn through a reservoir of molten nylon and then through a cooling zone to shrink the nylon on the unvulcanized hose. This was followed by vulcanization and stripping of the nylon sheath. Also known is a process for producing hose by the temporary application of a plastic jacket, the preferred jacket being disclosed as a polyamide. It was stated that the plastic jacket was hard enough coming out of the die to exercise pressure on the hose. It further stated the advantages of lighter weight and thinner walls and being able to put more hose on a vulcanization drum. The criteria for selecting such a plastic was given as the tensile strength, the temperature resistance, and hydrolysis resistance.
Other art discloses making of a tubular article where, prior to curing, the tube is wrapped with a shrinkable plastic made of Mylar and then heating to shrink it. Mylar is a trademark for an oriented polyethylene terephthalate. Other art discloses extruding a certain amount of a reusable material of appropriate thermal behavior for vulcanization purposes, to provide vulcanization energy and the shrinkage of the jacketing supplies the pressure. Further, it states that the overcoating is a thermoplastic shrinking composition while no specific materials are mentioned. Another patent discloses a process for curing an elastomeric tubular article having a thermoplastic jacket. In addition to the preferred materials being described as thermoplastic elastomers, which includes segmented polyesters such as Hytrel, the patent also cites the use of polyphenylenesulfide, polymethylpentene and ultra-high molecular weight polyethylene. It also discloses that the jacket serves to protect, compact, as well as yield a smooth tubular article surface during vulcanization.
Still another patent discloses a method for continuously vulcanizing hoses, wherein a layer of heat resistant synthetic resin, which has a melting point higher than the vulcanizing temperature, is provided over the outer periphery of a rubber hose in a uniform thickness. After the resin layer is cooled and hardened, the rubber hose is heated and cured in a microwave vulcanizing tank before being subjected to vulcanization by heat conduction in an ordinary vulcanizing tank. After completion of vulcanization, the synthetic layer of resin is peeled off and removed. Examples of the resin are described as being non-polar resins, and specifically mentioned are polytetrafluoroethylene (Teflon) or polymethylpentene. One object of the present invention is to provide a sheathing material which, when applied to a tubular article, can be quickly quenched to an amorphous state in order to preclude the distortion of the green tubular article prior to the vulcanization step. another object of the present invention is to provide a sheathing material which, when heated during the vulcanization step, is transformed from a lower density amorphous state to a higher density crystalline state in order to shrink upon the tubular article. A still further object of the present invention is to provide a sheathing material which may be cooled down after the vulcanization step to a brittle material which may be easily removed from the tubular article.